1. Field of the Invention
The present invention relates to a coded frame synchronizing method and circuit directed to coded information processed every frame such as voice codec and image codec.
In a multimedia multiplexing transmission system, it is required to pull transmit and receive operations in synchronization when a system is switched. Particularly, it is desired to establish the synchronization without any error with a time as short as possible. The voice codec and the image codec perform coding and decoding processes on the frame base. Hence, it is required that a frame synchronization be established between the coding and decoding parts. It is also required that a transmission error be detected and corrected in order to prevent the qualities of reproduced voice and image from being degraded.
2. Description of the Related Art
A synchronization code is added to coded information every frame in the voice codec and image codec in the multimedia multiplexing transmission system. Additionally, an error detection code or an error correction code may be added to the frame-base coded information. Parity bits function to detect a transmission error. A CRC (Cyclic Redundancy Check) code also functions to detect a transmission error. A BCH (Bose-Chaudhuri-Hocquenghen) code functions to correct a transmission error. The synchronization code is designed to be a frame synchronizing code having a unique pattern composed of a plurality of bits or a single-bit structure synchronization code and form a unique pattern over a plurality of frames.
On the receive side, the frame synchronization is established by detecting the synchronization code, and the coded information having a predetermined number of bits is extracted based on the detected synchronization code. Hence, the frame-based decoding process can be realized.
The coded frame has a length equal to, for example, 10 ms or 15 ms, and it is designed to reduce the number of times that the synchronization code is added in order to improve the transmission efficiency. The coded frame is sent by a transmission frame dependent on a transmission path such as a wire line or a radio channel. Hence, there are many cases where the coded frame differs from the transmission frame.
The coded information produced by the voice codec has a comparatively large correlation between the frames of coded information. If a transmission error occurs in the coded information of a coded frame, this coded frame is discarded, and the coded information of the previous coded frame is subjected to the reproduction process again. Hence, it is possible to suppress degradation of the quality of voice reproduced from coded information having a transmission error.
The error detection code such as the parity bits or the error correction code such as the BCH code is added in order to detect a transmission error in the coded information. The parity bits are added to the position of the synchronization bits, and the detection of the parity bits are detected by a parity check operation. Hence, the coded frame is pulled in synchronization (see, for example, Japanese Laid-Open Patent Application No. 62-53040). Another synchronizing method is known in which check bits are added to the coded frame instead of the synchronization code thereof and an error in the coded frame is detected (see, for example, Japanese Laid-Open Patent Application No. 63-1128).
As described above, the error detection code such as the parity bits or the error correction code such as the BCH code can be used as the synchronization code for the frame-base coded information. When the synchronization code is used, it is required to 1) pull the coded frame in synchronization as quickly as possible, 2) maintain the synchronized state without being affected by a fine transmission error, and 3) prevent occurrence of a pseudo-pulled-in-synchronization state.
In order to speed up the pulling-in operation, the synchronization code is generally designed to have a unique pattern which does not occur in any encoded information. However, the above synchronization code degrades the transmission efficiency. When the error detection code or the error correction code is used instead of the synchronization code as described above, degradation of the transmission efficiency can be prevented but there is a comparatively high possibility that the pseudo-pulled-in-synchronization state may occur. If the decoding process is carried out in the pseudo-pulled-in-synchronization state, the quality of reproduced voice and image will greatly be degraded.